complibs/isl: add HTTP mirror

Add well-known HTTP mirror as a fallback. This lets crosstool-ng
work when behind a HTTP/HTTPS only proxy.

Signed-off-by: Michael Hope <michaelh@juju.net.nz>
[me: split original patch in two]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <aeb4a850d0786ee62dc2.1375559989@wanda>
Patchwork-Id: 264436

reporter_name="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/"

reporter_comment="Toolchain for the Raspberry Pi, with hard-float."